Ames, Iowa — He wasn’t in the auditorium, but Texas Gov. Rick Perry loomed large over the eight candidates sparring at the GOP presidential debate Thursday night in Ames, Iowa.

Governor Perry is expected to announce his candidacy for president Saturday, a likely game-changer in the nascent 2012 race. The Texan brings to the table long executive experience, a record of job creation, populist charisma, and fundraising ability. He could give current (but weak) frontrunner Mitt Romney the kind of challenge the former governor of Massachusetts hasn’t faced yet from the already-announced candidates.

In fact, at Thursday’s debate, Mr. Romney stayed above the fray, as he had in the last debate, preferring to go after President Obama rather than any of the Republicans in the room. For Romney, the stakes were relatively low. He is not competing in Saturday’s straw poll – taking place at Iowa State University in Ames, like the debate – and thus faces no expectations of success.

But some of the other candidates must do well Saturday or see fundraising dry up, and the prospect of having to drop out. The Ames debate represented the last, best chance for the contenders to make a splash on TV before the straw poll, and they did not disappoint.

The two Minnesotans in the race – former Gov. Tim Pawlenty and Rep. Michele Bachmann – turned their simmering cold war into a hot one, leveling charges against one another over their records. Mr. Pawlenty portrayed Congresswoman Bachmann as an ideologue with nothing to show for it; Bachmann charged back with a litany of Pawlenty policies worthy of Mr. Obama.

“She's done wonderful things in her life, absolutely wonderful things,” said Pawlenty. “But it's an undisputable fact that in Congress, her record of accomplishment and results is nonexistent. That's not going to be good enough for our candidate for president of the United States.”

Bachmann, invited to respond, went after Pawlenty before defending herself. She began by accusing him of backing a “cap and trade” system to reduce greenhouse gas emissions, a policy now seen as anathema by most Republicans – and which Pawlenty has since disavowed.

“Governor,” she said, “when you were governor in Minnesota, you implemented cap-and-trade in our state, and you praised the unconstitutional individual mandate, and you called for requiring all people in our state to purchase health insurance that government would mandate. Third, you said the era of small government was over. That sounds a lot more like Barack Obama, if you ask me.”

Then Bachmann came to her own defense, arguing that she’s been fighting the good fight in Washington against raising the national debt ceiling, cap and trade, and “Obamacare.” In fact, she used versions of the word “fight” 11 times in the course of the evening, speaking crisply and punching back hard against the common criticism that the head of the House Tea Party Caucus is good at rabble-rousing but not at passing legislation.

“She said she's got a titanium spine,” said Pawlenty. “It's not her spine we're worried about. It's her record of results."

Bachmann called herself then-House Speaker Nancy Pelosi’s No. 1 target for defeat last year, “because I was effectively taking them on on nearly every argument they put forward.”

“I fought when others ran, I fought and I led against increasing the debt ceiling,” she said to cheers from the audience.

Bachmann and Pawlenty mud-wrestled with good reason: They both must do well at the straw poll or face diminished prospects, especially with Perry in the race. Bachmann is polling first among Iowa Republicans, so anything less than a first-place finish could damage her. The mild-mannered Pawlenty has underperformed since he entered the race, and has struggled for attention (and cash) especially since Bachmann entered. Still, Pawlenty has succeeded in keeping expectations low for Saturday, and, with a top-notch organization in Iowa, he could in fact win.

Another combative debater Thursday was former House Speaker Newt Gingrich, who was questioned about the mass exodus of his campaign staff in June and yawning campaign debt. Mr. Gingrich took offense, accusing the media of paying too much attention to “campaign minutiae” and engaging in “gotcha questions.”

Gingrich then invoked two previous GOP nominees, one of whom made it to the Oval Office despite campaign turmoil.

“Like Ronald Reagan,” he said, “who had 13 senior staff resign the morning of the New Hampshire primary and whose new campaign manager laid off a hundred people because he had no money because the consultants had spent it; like John McCain, who had to go and run an inexpensive campaign because the consultants spent it, I intend to run on ideas.”

On Iran, Congressman Paul argued against trade sanctions on Iran, repeating the libertarian argument for staying out of other countries’ internal business –even those that are developing nuclear weapons – and for generally getting out of the multiple US military engagements abroad.

“I say a policy of peace is free trade,” Paul said. “Stay out of their internal business. Don't get involved in these wars and just bring our troops home.”

Santorum jumped on that comment. “Iran is not Iceland, Ron,” he said. “Iran is a country that has been at war with us since 1979.”

Paul was also challenged over his position on marriage, which he believes should not be a federal issue. When asked if he believed it would be OK for a state to allow polygamy, Paul looked exasperated.

“Well, that's sort of like asking the question if the states wanted to legalize slavery, or something like that,” he said. “That is so past reality that no state is going to do that.”

Paul has a loyal core of supporters in Iowa and across the country, and could do well in the straw poll. Santorum has struggled to gain traction in the crowded field, and while he did claw his way to some attention in Ames, he could find it difficult to make it to the starting line of the nomination contests early next year, beginning in Iowa.

The other two candidates on the stage, businessman Herman Cain and former Utah Gov. Jon Huntsman Jr., struggled to make much of the debate. Mr. Cain made a splash in the first debate, in May, but has faded a bit since. For Mr. Huntsman, Thursday’s was his first debate and he could have done himself a favor by standing out. But he, like Romney and Gingrich, is not competing in the Ames straw poll, so the stakes aren’t as high on Saturday for him as they are for others.